Full-spectrum music tends to be a lot more fun to dance to, but bass in small bands can be tricky. The traditional approach is for someone to play a double bass, or more recently an electric one, but this means allocating an entire musician to the bass register. In a five-piece band a bass makes a lot of sense, and it can make sense in a four-piece band, but you need an amazing bass player [1] if you're going to be a trio. We only see someone playing a bass ~25% of the time (43/172) and only ~7% of trios (6/85) have a bass.

Another traditional approach is the piano, which lets a single musician play bass with one hand and chords or melody with the other. The piano is capable enough that even two-person bands, generally fiddle/piano, can put out a big full sound. [2] The biggest downside of the piano is that it doesn't have a way to produce crisp percussive high-end sounds. There are a bunch of things, however, that a third musician can do to provide this [3] and it's also possible for a rhythmic fiddler to take on a lot of it.

More recently many guitarists have been working with octave pedals. These are boxes (usually the BOSS OC-3) that duplicate the lowest notes of the instrument shifted down an octave or two. While you need to learn a somewhat different style of playing this allows a guitar player to simultaneously play bass. [4] At this point pretty much all the major bands that have guitar as their lowest instrument include octave pedals. [5]

Organ-style bass pedals seem like they could be useful here, since you can get stand-alone ones and they're designed for this. I don't know of any bands that use them, however. I've considered playing with them, but I generally want to use my feet for percussion. I'm also not sure how well they work for percussive as opposed to atmospheric bass.

With Kingfisher I've been experimenting with whistling bass lines while I play mandolin. It's still a work in progress, but I think it's promising. It does require being able to whistle accurately, clearly, and very loudly, however, which limits it's application. Possibly some tweaks to the algorithm could improve noise rejection enough that it would work with less strident input.

Various forms of droney bass can also work. Perpetual eMotion would use a didgeridoo, Owen Marshall has been known to put stones on harmonium keys and pump with his foot, and I've used my breath controller to pulse single notes jawharp-style. Since you only have a single pitch these are relatively limited, but this also means they don't take much to operate.

With electronic instruments and sensors generally making experimentation easier here, I'm curious to see what else people try.

[1] Example: the Stringrays when they play as a three-piece, primary instrumentation fiddle/guitar/bass.

[2] Example: Buddy System, primary instrumentation fiddle/feet/piano.

[3] Example: the Free Raisins, primary instrumentation fiddle/mandolin/piano.

[4] Example: Maivish, primary instrumentation fiddle/guitar (two-piece) or fiddle/flute/guitar (three piece).

[5] Counterexample: Bethany Waickman's bands, though since she plays in DADGAD, mics the guitar in a bassy way, and has a generally bassy playing style she doesn't really need one!